The 8051 operates based on an external crystal. This is an electrical device which, when energy is applied, emits pulses at a fixed frequency. One can find crystals of virtually any frequency depending on the application requirements. When using an 8051, the most common crystal frequencies are 12 megahertz and 11.059 megahertz--with 11.059 being much more common. Why would anyone pick such an odd-ball frequency? There’s a real reason for it--it has to do with generating baud rates and we’ll talk more about it in the Serial Communication chapter. For the remainder of this discussion we’ll assume that we’re using an 11.059Mhz crystal.

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